The thermal stability of nitrogen in physical vapor deposited (PVD) HfO xN y gate dielectrics with different nitrogen concentrations was investigated. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and electrical characterization techniques were used for the investigation. The excellent electrical stability of reactive sputtered HfO xN y gate dielectrics during the post deposition annealing was due to nitrogen which at the HfO xN y/Si interface formed N-Si bonds. The results show that during the postdeposition annealing the bulk Hf-N bonds in the reactive-sputtered HfO xN y are not stable and can be easily replaced by oxygen.